[ADD] purchase: How to setup two levels of approval for purchase orders?

https://docs.google.com/document/d/1uyrkspB9NwYb49zsO8pt5HhWZ4cPJiFdTvwqqSis37E/edit

purchase/purchases/rfq/approvals.rst

@@ -2,4 +2,33 @@
 How to setup two levels of approval for purchase orders?
 ========================================================
 
-*Coming soon...*
+Two level approval setup
+========================
+
+Double validation on purchases forces a validation when the purchased amount
+exceeds a certain limit.
+
+Install **Purchase Management** module and then go to **General Settings** to
+configure the company data.
+
+.. image:: media/approvals01.png
+    :align: center
+
+Set here the amount limit for second approval and set approval from
+manager side.
+
+Process
+=======
+
+Logged as a purchase user, create a purchase order for more than the amount set
+above, and confirm it. The purchase order is set in a state **To Approve**
+
+.. image:: media/approvals02.png
+    :align: center
+
+The manager gets the order to approve and validates the final order.
+
+.. image:: media/approvals03.png
+    :align: center
+
+Once approved, the purchase order follows the normal process.